This invention relates to a diamond sinter produced by sintering a mixture containing at least coated diamond particles having a coat forming substance formed on the surfaces of diamond particles and a high-pressure phase boron nitride sinter produced by sintering a mixture containing at least coated high-pressure phase boron nitride particles having a coat forming substance formed on the surfaces of high-pressure phase boron nitride particles. The invention also relates to processes for producing those sinters.
Diamond, with strong covalent bonding, has many excellent properties. On the other hand, because of this covalent bonding, diamond has such a small self-diffusion coefficient that it is very difficult to sinter. Another problem with diamond is that it is thermodynamically stable only under ultrahigh pressures so that it will experience a phase transition to graphite if the pressure is insufficient under elevated temperatures. Therefore, without addition of bonding materials or sintering aids, strong sinters cannot be produced unless a ultrahigh pressure of about 8500 MPa is applied simultaneously with exposure to a very high temperature of about 2440K.
High-pressure phase boron nitrides consist of cubic boron nitride and/or wurtzite boron nitride. Cubic boron nitride (c-BN) has similar characteristics to diamond; on account of strong covalent bonding, it has many excellent properties but, on the other hand, because of this covalent bonding, c-BN has such a small self-diffusion coefficient that it is very difficult to sinter. Further, c-BN is thermodynamically stable only under ultrahigh pressures and it will experience a phase transition to hexagonal boron nitride (h-BN) of a graphitic phase (if the pressure is insufficient under elevated temperatures.) Wurtzite boron nitride (w-BN) has generally the same properties as the cubic boron nitride; it is a difficult-to-sinter substance and it is thermodynamically stable only at ultrahigh pressures and will experience a phase transition to hexagonal boron nitride (h-BN) of a graphitic phase if the pressure is insufficient under elevated temperatures. It should be noted that if the wurtzite boron nitride is sintered under ultrahigh pressures and at elevated temperatures where the cubic boron nitride is thermodynamically stable, a phase transition to the cubic boron nitride occurs depending on the conditions, yielding in this case a sinter containing the cubic boron nitride or a sinter that is solely comprised of the cubic boron nitride. In either case, without addition of bonding materials or sintering aids, strong sinters cannot be produced unless a ultrahigh pressure of about 7000 MPa is applied simultaneously with exposure to a very high temperature of about 2000K.
However, irrespective of whether diamond or high-pressure phase boron nitride is to be sintered, the sintering conditions set forth in the preceding paragraphs for the case where neither bonding materials nor sintering aids are added are extremely hostile and are not suitable for the purpose of producing diamond or high-pressure phase boron nitride sinters of high density on an industrial scale.
Therefore, in order to produce these high-density sinters, sintering must be done under industrially applicable conditions and to this end the addition of bonding materials and/or sintering aids is essential.
The first thing to be stated in this connection is that in recent years, a particular need exists for enhancing the performance of diamond or high-pressure phase boron nitride sinters of high density and to this end, it is required that diamond or high-pressure phase boron nitride particles such as those not larger than 10 .mu.m be used as the starting material and sintered to produce a diamond or high-pressure phase boron nitride sinter of high density.
Conventionally, diamond or high-pressure phase boron nitride sinters have been produced mainly by a method in which bonding materials and/or sintering aids are added in a powder form. In this case, even if the bonding materials and/or sintering aids are of a fine particulate form, it is extremely difficult to insure ideal uniform addition or uniform dispersion in such a way that they cover all of the individual diamond or high-pressure phase boron nitride particles present. Even if such uniform dispersion is realized, the extent of "uniformity" that can be attained is limited since the bonding materials and/or sintering aids are added on a particle basis. It should particularly be noted that if the bonding materials or sintering aids are added in small quantities, the resulting sinter will inevitably contain those areas where the bonding materials or sintering aids are absent.
In many actual cases, those powder particles or the particles of the bonding material and/or sintering aid agglomerate to either form lumps in the sinter or occur unevenly in the sinter. Hence, particularly in those areas where diamond or high-pressure phase boron nitride particles agglomerate, the structure is essentially the same as what is produced without addition of bonding materials or sintering aids and unsintered portions will locally occur. In those areas where the bonding materials and/or sintering aids agglomerate, the microscopic structure contains portions where diamond or high-pressure phase boron nitride particles are absent from the sinter. Either phenomenon produces imperfections that cause marked deterioration in the performance of the produced diamond or high-pressure phase boron nitride sinter.
Therefore, in order to produce diamond or high-pressure phase boron nitride sinters of high performance that are free from the above-mentioned imperfections, it is necessary that the bonding material and/or sintering aid be positively distributed on individual diamond or high-pressure phase boron nitride particles and, to this end, it is strongly desired to produce diamond or high-pressure phase boron nitride sinters of high performance using coated diamond or high-pressure phase boron nitride particles which are prepared by coating uniformly a bonding material forming substance and/or a sintering aid forming substance on the individual particles of diamond or high-pressure phase boron nitride, respectively.
Secondly, as already mentioned, both diamond and high-pressure phase boron nitride are substances that are extremely difficult to sinter and which are stable only under ultrahigh pressures and, hence, in order to prevent the diamond or high-pressure phase boron nitride from experiencing a phase transition to the lower-pressure phase (graphite or h-BN) and to produce a dense sinter of the diamond or high-pressure phase boron nitride, manufacturers have adopted the practice of using bonding materials and/or sintering aids under ultrahigh pressures in excess of 2000 MPa and at elevated temperatures. According to the report of T. Noma et al. (J. Master. Scie., 19 (1984) 2319-2322), diamond is sintered at 6000 MPa (60 kb) and at 1300.degree. C. or above. Cubic boron nitride may be sintered at 4000 MPa (40 kb) and at 1200.degree. C. or above, as taught in Unexamined Published Japanese Patent Application (kokai) Sho 63-35456. These sintering conditions refer to extreme states and they are both rigorous indeed in that they cannot be created without using a ultrahigh pressure apparatus such as of the girdle or belt type.
This constraint of using a ultrahigh pressure apparatus such as of the girdle or belt type for creating ultrahigh pressures in excess of 2000 MPa has made it difficult to achieve mass production of diamond or high-pressure phase boron nitride sinters and not only has the production cost been high but it has also been impossible to manufacture large shapes.
However, speaking of diamonds, Hall conducted an experiment under ultrahigh pressures and reported that when a diamond was thermodynamically metastable if not stable, it took such a long time for a phase transition to occur that it remained stable for practical purposes; he then drew line 2 as shown in FIG. 33a to indicate the upper limit beyond which the diamond would no longer exist in a practically stable manner (H. T. Hall, Science, 169 (1970) 868-869). Hence, according to his report, even under low-pressure and high-temperature conditions that are not stable with respect to thermodynamic equilibrium line 1 in FIG. 33a, a diamond will exist practically stable if it is at temperatures that do not exceed line 2 in FIG. 33a, say, up to about 1400 K (ca. 1100.degree. C.), given a pressure of up to about 35 kb.
Speaking of high-pressure phase boron nitrides, Wakatsuki et al. conducted an experiment on a cubic boron nitride under ultrahigh pressures and reported that when the cubic boron nitride was thermodynamically metastable if not stable, it took such a long time for a phase transition to occur that it remained stable for practical purposes; they then drew line 2 as shown in FIG. 33b to indicate the upper limit beyond which the cubic boron nitride would no longer exist in a practically stable manner (Wakatsuki, Ichise, Aoki and Maeda, Abstracts of the Lectures Read at the 14th Symposium on High-Pressure Technology (1972) p. 78). Hence, according to their report, even under low-pressure and high-temperature conditions that are not stable with respect to thermodynamic equilibrium line 1 in FIG. 33b, a cubic boron nitride will exist practically stable if it is at temperatures that do not exceed line 2 in FIG. 33b, say, not higher than 1200.degree. C.
Based on these facts, one may conclude that if diamond sinters containing diamonds or high-pressure phase boron nitride sinters containing high-pressure phase boron nitrides at least one of which is a cubic boron nitride can be produced under the stable conditions mentioned above, he can effectively use a ultrahigh pressure apparatus such as a piston-cylinder (PC) type ultrahigh pressure apparatus that is capable of creating comparatively moderate ultrahigh pressures less than 2000 MPa, or a ultrahigh pressure HIP apparatus as a known technique capable of pressurization up to 1000 MPa, or hot isostatic press (HI) apparatus excluding the ultrahigh pressure HIP apparatus, or a hot press (HP) apparatus.
Thusly, unlike in the case of ultrahigh pressure apparatus of such types as the aforementioned girdle and belt types that create ultrahigh pressures in excess of 2000 MPa, not only does it become easy to achieve mass production but it also becomes possible to produce sinters of large shapes.
In recent years, it is strongly desired that diamond or high-pressure phase boron nitride sinters of high density and performance that contain comparatively large amounts of diamond or high-pressure phase boron nitride particles such as those not larger than 10 .mu.m should be manufactured in high volumes or in large shapes using a ultrahigh pressure apparatus such as the aforementioned piston-cylinder (PC) type ultrahigh pressure apparatus which is capable of creating comparatively moderate ultrahigh pressures less than 2000 MPa, or the ultrahigh pressure HIP apparatus capable of pressurization up to 1000 MPa, the hot isostatic press (HIP) apparatus excluding the ultrahigh pressure HIP apparatus, or the hot press (HP) apparatus.
In fact, however, it is not easy to assure that diamond or high-pressure phase boron nitride sinters containing fairly large amounts of diamond or high-pressure phase boron nitride, respectively, are effectively densified under those limited pressures.
Stated more specifically, this can be realized only when the bonding material which is added in the form of a feed powder for that material deforms or migrate during sintering to fill the gaps between diamond or high-pressure phase boron nitride particles so that the applied pressure will work effectively for the displacement and loss of pores or the voids which are surrounded by the bonding material and the diamond or high-pressure phase boron nitride particles. To this end, the bonding material and/or the feed powder for that material which reacts with diamond or high-pressure phase boron nitride to generate the bonding material must be present in such a way that they contact and surround the diamond or high-pressure phase boron nitride particles.
It should particularly be noted that under the sintering pressure and temperature conditions where the pressure is less than 2000 MPa and where diamond or high-pressure phase boron nitride is thermodynamically metastable if not stable, there is little chance for the particles of diamond or high-pressure phase boron nitride to bind directly and, hence, those portions of the diamond or high-pressure phase boron nitride particles in the resulting sinter where the feed powder for the bonding material is absent will remain unsintered. As a result, the performance of the sinters produced will deteriorate considerably. If the bonding material is added only in the form of a feed powder for that material, it is extremely difficult to insure ideal uniform addition or uniform dispersion in such a way that the particles of the feed powder cover all of the individual diamond or high-pressure phase boron nitride particles present even if the particles of the .feed powder are fine. Even if such uniform dispersion is realized, the extent of "uniformity" that can be attained is limited since the feed powder for the bonding material is added on a particle basis. Thus, a serious problem is presented in the manufacture of diamond or high-pressure phase boron nitride sinters that contain large amounts of diamond or high-pressure phase boron nitride, respectively. Therefore, even in the case of mixing diamond or high-pressure phase boron nitride particles with the feed powder for the bonding material, one may compensate for the insufficiency by using coated diamond or high-pressure phase boron nitride particles which are prepared by coating the surfaces of bare diamond or high-pressure phase boron nitride particles with a bonding material forming substance that is of the same and/or different type from the feed powder for the bonding material; as a consequence, the distribution of the bonding material can be made sufficiently uniform. Hence, the coating method for preparing such coated diamond or high-pressure phase boron nitride particles bears technologically great importance.
Stated more specifically, irrespective of whether the bonding material is added in the form of a feed powder for bonding material forming substance or not, the bonding material and/or a substance that reacts with diamond or high-pressure phase boron nitride to form the bonding material is allowed to be present covering the surfaces of diamond or high-pressure phase boron nitride particles. Only by following this approach, one can assure that diamond or high-pressure phase boron nitride sinters of high density and performance that contain comparatively large amounts of diamond or high-pressure phase boron nitride particles are manufactured in high volumes or large shapes using a ultrahigh pressure apparatus such as the piston-cylinder (PC) type ultrahigh pressure apparatus which is capable of creating comparatively moderate ultrahigh pressures less than 2000 MPa, or the ultrahigh pressure HIP apparatus which is capable of pressurization up to 1000 MPa, or the hot isostatic press (HIP) apparatus excluding the ultrahigh pressure HIP apparatus, or the hot press (HP) apparatus.
While the foregoing description is directed to diamond or high-pressure phase boron nitride sinters that use the respective particles with a particle diameter not greater than 10 .mu.m, it should be noted that if the sinters are to be used in definite applications, it is very effective to intentionally use diamond or high-pressure phase boron nitride particles of comparatively large particle diameters, say, in excess of 10 .mu.m.
Consider, for example, a wear-resistant sinter that takes advantage of the ultrahigh hardness of diamond or high-pressure phase boron nitride. This sinter can be produced by sintering a dispersion of a comparatively large quantity of diamond or high-pressure phase boron nitride particles in excess of 10 .mu.m in diameter and, in this case, using diamond or high-pressure phase boron nitride particles larger than 10 .mu.m in diameter as a starting material is extremely important.
The diamond or high-pressure phase boron nitride sinter which has dispersed therein diamond or high-pressure phase boron nitride particles exceeding 10 .mu.m in diameter is characterized in that the effect of dispersion of the dispersed particles is marked because those dispersed particles are sintered as they are closely bound to the surrounding microstructure without imperfections or pores. However, as already mentioned, both diamond and high-pressure phase boron nitride are extremely difficult to sinter and, therefore, it is essential to use a sintering aid or a bonding material that accelerate the sintering of the diamond or high-pressure phase boron nitride particles larger than 10 .mu.m in diameter together with the surrounding microstructure.
Conventionally, such sintering aids and bonding materials have been added solely by the powder mixing method. However, the incorporation of impurities during mixing is unavoidable in this method; what is more, there is theoretically a limit on the extent to which a uniform structure can be realized. Even if the particles of a sintering aid or a bonding material are extremely fine, it is very difficult to insure ideal uniform mixing or uniform dispersion in such a way that the particles of a sintering aid or bonding material powder cover all of the diamond or high-pressure phase boron nitride particles present. Even if such uniform dispersion is realized, the extent of "uniformity" that can be attained is limited since the particles of the sintering aid or bonding material powder are mixed on a particle basis. Particularly in the case where those particles are used in relatively small amounts, an uneven distribution will occur inevitably.
In many actual cases, the dispersed diamond or high-pressure phase boron nitride particles lump together or the particles of the sintering aid or bonding material powder agglomerate, as a result, the diamond or high-pressure phase boron nitride particles will form lumps in the sinter or they occur unevenly in the diamond or high-pressure phase boron nitride sinter, eventually leading to marked deterioration in the performance of the sinters.
Therefore, it is necessary that the sintering aid or the bonding material be distributed positively to all individual diamond or high-pressure phase boron nitride particles exceeding 10 .mu.m in diameter. Further, in order to assure that the diamond or high-pressure phase boron nitride particles are sintered as they are closely bound to the surrounding microstructure, it is required to provide a highly controlled uniform coating on the surfaces of the diamond or high-pressure phase boron nitride particles; the coating is uniform in that it leaves no surfaces of the individual diamond or high-pressure phase boron nitride particles left uncovered and it is highly controlled in that this uniform coating is applied to all individual diamond or high-pressure phase boron nitride particles. What is more, the greater the size of these particles, the more uniform this highly controlled coating should be in order to further reduce the uncovered portions of the particles.
Thus, the production of coated diamond or high-pressure phase boron nitride particles of desired diameters in which diamond or high-pressure phase boron nitride particles are covered with uniform coatings that are highly controlled in accordance with the particle diameter, as well as the manufacture of high-performance sinters using those coated particles are strongly desired.
While coat forming substances could be applied by various techniques such as vapor-phase processes and wet plating methods, the vapor-phase approach in which inorganic materials, metallic materials and other coat forming substances are applied to provide coatings as films and various other forms theoretically have major features that are unattainable by other coating techniques, such as: (1) easy control of the atmosphere; (2) the selection of coat forming substances is basically unlimited and various kinds of substances including elemental metallic substances (e.g. active metals), alloys, nitrides, carbides, borides and oxides can be applied; (3) the desired substance can be applied in high purity; and (4) the coating weight of the coat forming substance can be controlled freely.
However, for the reasons to be set forth below, none of the various coating apparatus and methods heretofore proposed as known techniques have been capable of forming the above-described highly controlled uniform coatings.
Particularly in the case of diamond or high-pressure phase boron nitride particles not larger than 10 .mu.m, the diamond or high-pressure phase boron nitride particles to be coated are so fine that they are cohesive enough to have a great tendency to agglomerate together, whereby almost all single particles form agglomerates. Since these agglomerates cannot be disintegrated unless they are subjected to a special action greater than their cohesive force, they cannot be simply coated as such to insure that the surfaces of the individual particles are covered with the coating of a bonding material and/or sintering aid, eventually yielding coated agglomerates in which the surfaces of the agglomerates are covered with the coating of the bonding material and/or sintering aid. This has caused a problem with the individual diamond or high-pressure phase boron nitride particles forming the agglomerates in that the surfaces of the particles located on the surfaces of the agglomerates have large coating weights but suffer from uneven coating whereas the particles located within the agglomerates are not covered at all.
With a view to solving these problems, attempts have already been made to coat the particles in a dispersed state in order to assure the coating of the individual particles in the powder of the core particles to be coated. However, for the reasons stated below, none of the various coating apparatus and methods heretofore proposed as known techniques have been capable of forming the intended uniform coating.
For instance, Unexamined Published Japanese Patent Application (kokai) Sho 58-31076 teaches an apparatus and method, according to which a vessel placed in PVD equipment is charged with the particles in a powder of core particles and vibrated by an electromagnetic means so that the core particles in the vessel are rolled as they are coated by a PVD process. Unexamined Published Japanese Patent Application (kokai) Sho 61-30663 teaches an apparatus, according to which a vessel placed in PVD equipment is charged with the particles in a powder of core particles and vibrated by a mechanical means so that the core particles in the vessel are rolled as they are coated by a PVD process. However, in the actual practice with those apparatus or methods in which the vessel is vibrated so that the particles in the powder of core particles are rolled as they are provided with coatings, the necessary action for disintegrating the agglomerates of diamond or high-pressure phase boron nitride particles with an average diameter not greater than 10 .mu.m by applying a force exceeding their cohesive force cannot be produced and, hence, the agglomerates cannot be disintegrated; to the contrary, a granulating action develops to form agglomerates that are greater in number or size than before the powder of core particles is supplied into the vessel. On the other hand, diamond or high-pressure phase boron nitride particles having an average diameter in excess of 10 .mu.m are simply subjected to a sliding action as they form many layers in superposition and it has been impossible to achieve the desired coating of single separate particles.
Unexamined Published Japanese Patent Application (kokai) Hei 3-153864 teaches an apparatus and method, according to which a rotating vessel having barriers and/or ridges and grooves in the inner surface is charged with core particles and rotated as the surfaces of the particles are coated by an evaporation method. The problem with this apparatus and method is that the necessary action for disintegrating the agglomerates of diamond or high-pressure phase boron nitride particles with an average diameter not greater than 10 .mu.m by applying a force exceeding their cohesive force cannot be produced and, hence, the agglomerates cannot be disintegrated and, what is more, an increased number or size of agglomerates will simply form. On the other hand, diamond or high-pressure phase boron nitride particles having an average diameter in excess of 10 .mu.m are simply subjected to a gentle stirring action as many of them contact one another forming many layers in superposition and it has been impossible to achieve the desired coating of single separate particles.
Unexamined Published Japanese Patent Application (kokai) Sho 58-141375 teaches an apparatus in which the particles of a powder in a reactive gas atmosphere are suspended by the flow of the reactive gas under gravity and in which the surfaces of the particles are coated with the precipitating substance that forms by the chemical reaction involving the reactive gas. Unexamined Published Japanese Patent Application (kokai) Hei 2-43377 teaches a method in which particles placed under vacuum are fluidized as they are subjected to coating by a thermochemical reaction treatment. Unexamined Published Japanese Patent Application (kokai) Sho 64-80437 teaches a method in which the agglomerates of core particles in powder are disintegrated by a sound wave that is a composite of low and high frequency waves, so that the agglomerates are fluidized to improve the coating efficiency. However, these techniques which utilize the fluidized bed of the particles in a powder of core particles which is formed by a gas flow or vibrations have had the problem that with diamond or high-pressure phase boron nitride particles having an average diameter of no more than 10 .mu.m, it is practically impossible to fluidize the separate individual particles, thus failing to disintegrate the agglomerates of these particles. On the other hand, with diamond or high-pressure phase boron nitride particles exceeding 10 .mu.m in average diameter, it is practically impossible to insure that all of these particles are similarly and independently fluidized and suspended as single separate entities and one has been incapable of eliminating uneven coating of the particles which is due to the hiding of one particle by another.
Unexamined Published Japanese Patent Application (kokai) Sho 54-153789 teaches an apparatus in which a powder material is dropped within a vacuum vessel, where the metal vapor is generated to form a metal coating on the particles. Unexamined Published Japanese Patent Application (kokai) Sho 60-47004 teaches a method in which a monomer gas and the particles of a powder are introduced into a high-frequency plasma region in a vacuum vessel, where a coating film of an organic substance is formed by plasma-assisted polymerization. If diamond or high-pressure phase boron nitride particles with an average diameter of no more than 10 .mu.m are simply introduced as in the techniques described above, agglomerates cannot be disintegrated. On the other hand, diamond or high-pressure phase boron nitride particles exceeding 10 .mu.m in average diameter will simply drop while forming agglomerates which are not single separate particles and various problems occur, such as uneven coating due to the hiding of one particle by another, the total failure of the particles within an agglomerate to be coated, and differences in the coating weights of individual particles.
Unexamined Published Japanese Patent Application (kokai) Sho 64-80437 teaches a method in which the agglomerates of core particles in powder are disintegrated by a sound wave consisting of low and high frequency waves so that they are fluidized to improve the coating efficiency. However, this method which imparts vibrations to the fluidized bed has had the problem that with diamond or high-pressure phase boron nitride particles having an average diameter of no more than 10 .mu.m, it is practically impossible to fluidize the separate individual particles, thus failing to disintegrate the agglomerates of these particles. On the other hand, with diamond or high-pressure phase boron nitride particles exceeding 10 .mu.m in average diameter, it is practically impossible to insure that all of these particles are similarly and independently fluidized and suspended as single separate entities and one has been incapable of eliminating uneven coating of the particles which is due to the hiding of one particle by another.
Unexamined Published Japanese Patent Application (kokai) Sho 62-250172 teaches an apparatus and method, according to which a powder that has been preliminarily treated by jet milling is allowed to stay within a chamber for heat treatment under vacuum, where it is subjected to a heat treatment and thence dropped under gravity through a powder feeder into a cylinderal sputtering chamber equipped with a vertical target, whereby the particles in powder are provided with a coating. Unexamined Published Japanese Patent Application (kokai) Hei 2-153068 teaches an apparatus and method, according to which a powder that has been preliminarily treated by jet milling is allowed to stay within a chamber for heat treatment under vacuum, where it is subjected to a heat treatment and thence introduced through a powder feeder into a rotary vessel accommodating a sputter source within a sputtering chamber in the form of a powder (not as single particles), with sputtering being effected as the vessel is rotated. These techniques involve a heating step which is performed before coating so that the jet-milled powder of core particles is allowed to stay for heat treatment and because of this staying of the powder in the heating step, the diamond or high-pressure phase boron nitride particles of any diameter will form agglomerates again which are not single particles and, eventually, such agglomerates will not revert to single particles in the coating step.
Thus, none of the so far proposed techniques have successfully solved the problems associated with the apparatus or method for providing coatings on the core particles in powder which are diamond or high-pressure phase boron nitride particles. In actual cases, diamond or high-pressure phase boron nitride particles having an average diameter of not more than 10 .mu.m form agglomerates which cannot be disintegrated and, hence, no methods or apparatus have been available for producing coated diamond or high-pressure phase boron nitride particles in which said diamond or high-pressure phase boron nitride particles, being dispersed as single particles, are covered on their surfaces with coat forming substances including a bonding material forming substance and/or a sintering aid forming substance.
Speaking of diamond or high-pressure phase boron nitride particles exceeding 10 .mu.m in average diameter, these particles are in actual cases subjected to a coating treatment in the form of agglomerates in which they remain in mutual contact and, hence, those portions which are blocked by other particles remain uncoated. As already mentioned, highly controlled uniform coatings are needed and although even the small cohesive force discussed above has been so much influential as to cause a very serious problem, no methods have been available for producing the satisfactory coated diamond or high-pressure phase boron nitride particles, nor have been apparatus for implementing such methods.
In short, irrespective of the diameter of starting diamond or high-pressure phase boron nitride particles, it has been impossible to prepare coated diamond or high-pressure phase boron nitride particles by providing each of those starting particles with a controlled uniform coat by vapor-phase coating techniques using coat forming substances including bonding material forming substances and/or sintering aid forming substances.
Hence, it has been impossible to manufacture diamond or high-pressure phase boron nitride sinters of high performance that are uniform, dense and firmly sintered and which have a highly controlled microstructure using small (e.g., .ltoreq.10 .mu.m) coated diamond or high-pressure phase boron nitride particles. In addition, diamond or high-pressure phase boron nitride sinters of high density and performance that contain comparatively large amounts of diamond or high-pressure phase boron nitride based on those coated particles could not be manufactured in high volumes or large shapes. Further, diamond or high-pressure phase boron nitride sinters of high density and performance that contain comparatively large amounts of diamond or high-pressure phase boron nitride as derived from a mixture of those coated particles and a separately added bonding material could not be manufactured in high volumes or large shapes.
It has also been impossible to manufacture wear-resistant sinters that have dispersed therein comparatively large amounts of coated diamond or high-pressure phase boron nitride particles exceeding, for example, 10 .mu.m in diameter.